Cesium-lithium-borate crystal and its application to frequency conversion of laser light

ABSTRACT

The present invention provides a cesium-lithium-borate crystal, which can be used as a high-performance wavelength converting crystal, having a chemical composition expressed as CsLiB6O10, and substituted cesium-lithium-borate crystals expressed by the following formula:or(where, M is an alkali metal element, and L is an alkali earth metal element); a method for manufacturing same by heating and melting; and an optical apparatus using such crystals

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cesium-lithium-borate crystal andcrystals with substituted chemical compositions thereof. Moreparticularly, the present invention relates to a cesium-lithium-boratecrystal and crystals with substituted chemical compositions thereofwhich are used as frequency converting nonlinear optical crystals inlaser oscillators and optical parametric oscillators used in blue andultraviolet lithography, laser microprocessing, scientific andindustrial measurement and laser nuclear fusion, a method formanufacturing same, and an optical apparatus using same.

2. Prior Art and Problems

Laser oscillators used in ultraviolet lithography, lasermicroprocessing, scientific and industrial measurement and laser nuclearfusion must generate stable blue and ultraviolet rays efficiently. Oneof the methods to achieve the above object which is attracting thegeneral attention to day is the method to efficiently obtain blue andultraviolet rays by frequency conversion of a light source usingnonlinear optical crystals.

A pulse YAG laser oscillator, a type of laser oscillators, for example,uses nonlinear optical crystals to convert frequency of a light sourceto generate the third (wavelength: 355 nm) or the fourth (wavelength:266 nm) harmonics of the pulse YAG laser.

Many contrivances on the frequency converting none linear opticalcrystals which are indispensable for generating ultraviolet rays havebeen announced. For example, beta barium methaborate (β-BaB₂O₄), lithiumtriborate (LiB₃O₅), cesium triborate (CsB₃O₅) and other borate crystalsare known. These frequency converting nonlinear optical crystals forgenerating blue and ultraviolet rays will pass wavelengths of 200 nm andbelow, and have a large nonlinear optical coefficient.

It is very difficult, however, to grow crystals of β-BaB₂O₄, one of suchfrequency converting nonlinear optinal crystals, because of the tendencyof causing phase transition in the production process. Further, angularallowance is very tight, and thus this particular substance has a verylow level of generality.

Furthermore, for LiB₃O₅ other frequency converting nonlinear opticalcrystal, the growth time is very long as a result of flux growth in theproduction process, and this crystal is only good for phase matching forrays down to about 555 nm wavelengths for second harmonic generation.This crystal is used, for example, for generation of the third harmonic(wavelength: 355 nm) of Nd-YAG lasers, but cannot be used for generatingthe fourth harmonic (wavelength: 266 nm)

SUMMARY OF THE INVENTION

The present invention was developed to overcome the above-mentioneddrawbacks of the prior art and has an object to provide acesium-lithium-borate crystal and crystals with substituted chemicalcompositions which are high-performance frequency converting nonlinearoptical crystals that pass shorter wavelengths, have a high conversionefficiency, and have a large temperature and angular allowance, a methodfor manufacturing such crystals, and a method for utilizing same.

As means to solve the above-mentioned problem, the present inventionprovides a cesium-lithium-borate crystal having a chemical compositionexpressed as CsLiB₆O₁₀.

The present invention provides also a substituted cesium-lithium-boratecrystal having a chemical composition expressed by the followingformula:

Cs_(1−x)Li_(1−y)M_(x+y)B₆O₁₀

(where, M is at least one alkali metal element other than Cs and Li, andx and y satisfy the relationship of 0≦x≦1 and 0≦y≦1, and x and y nevertake simultaneously a value of 0 or 1), or the following formula

Cs_(2(1−z))Li₂L_(z)B₁₂O₂₀

(where, L is at least one alkaline earth metal element, and 0<z<1)

Furthermore, the present invention provides also a method formanufacturing the above-mentioned crystals by heating and melting a rawmaterial mixture of constituent elements, a method for manufacturing theabove-mentioned crystals through growth by the melt methods comprisingthe crystal pulling method and top seeded kyropoulos method, and amethod for manufacturing the above-mentioned crystals through growth bythe flux method.

The present invention furthermore provides a frequency converter and anoptical parametric oscillator provided with the abovecesium-lithium-borate crystal or the crystal With a substituted chemicalcomposition as optical means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structural view illustrating an example of structure of afive-zone furnace for growing a cesium-lithium-borate crystal which isan embodiment of the present invention.

FIG. 2 shows a three-dimensional structural diagram illustrating thestructure of the cesium-lithium-borate crystal of the present invention.

FIG. 3 shows a graph illustrating transmission spectrum of thecesium-lithium-borate crystal of the present invention.

FIG. 4 shows a graph illustrating the refractive index dispersion curvefor the cesium-lithium-borate crystal of the present invention.

FIG. 5 shows a relational diagram illustrating the relationship betweenthe mixing ratio of boron oxide (B₂O₃) and temperature duringmanufacture of the cesium-lithium-borate crystal of the presentinvention.

FIG. 6 shows a graph illustrating the relationship between the phasematching angle θ and the incident laser wavelength of a crystal capableof generating the second harmonic (SHG) in Nd:YAG laser with thecesium-lithium-borate crystal which is an embodiment of the presentinvention.

FIG. 7 shows a graph illustrating the relationship between the walk-offangle and the incident laser wavelength in Nd:YAG laser of acesium-lithium-borate crystal which is an embodiment of the presentinvention.

FIG. 8 shows a graph illustrating the fourth harmonic generatingcharacteristics of a cesium-lithium-borate crystal which is anembodiment of the present invention.

FIG. 9 shows a graph illustrating a theoretical curve of non-criticalphase matching wavelength resulting from generation of a sum frequency,and wavelength of the resultant sum frequency for acesium-lithium-borate crystal which is an embodiment of the presentinvention.

FIG. 10 is a photograph taking the place of a drawing of the fifthharmonic, and the second and fourth harmonics of Nd:YAG laser availableby the generation of sum frequency for a cesium-lithium-borate crystalwhich is an embodiment of the present invention.

FIG. 11 shows a schematic representation of the beam pattern of thefifth harmonic, and the second and fourth harmonics of Nd:YAG laseravailable by the generation of sum frequency for a cesium-lithium-boratecrystal which is a embodiment of the present invention.

FIG. 12 shows a graph illustrating the phase matching tuning curve ofType-I OPO using a cesium-lithium-borate crystal which is an embodimentof the present invention with wavelengths of the excited light of 213nm, 266 nm, 355 nm and 532 nm.

FIG. 13 shows a A graph illustrating the phase matching tuning carve ofType-II OPO using a cesium-lithium-borate crystal which is an embodimentof the present invention with a wavelength of the excited light of 532nm; and

FIG. 14 shows a graph illustrating power X-ray diffraction dataregarding Cs_(1−x)LiRb_(x)B₆O₁₀ crystal.

DETAILED DESCRIPTION OF THE INVENTION

The present inventor noticed the fact that the borate crystals such asbeta-barium methaborate (β-BaB₂O₄), lithium triborate (LiB₃O₅), andcesium triborate (CsB₃O₅), which are conventionally used as frequencyconverting nonlinear optical crystals for generating blue andultraviolet rays, were generally borate crystals containing independentmetals, and found that high-performance borate crystals never seenbefore can be realized by adding a plurality or kinds of metal ions.

The present inventor produced several kinds of borate crystalscontaining ions of two or more kinds of metals such as an alkali metaland an alkaline earth metal, and irradiated them with an Nd YAG laser(wavelength: 1,064 nm) to generate the second harmonic (wavelength: 532nm) in order to find an optimum metal combination through a number ofexperimental verifications.

As a result, he has found that the borate crystals containing both Csand Li, in particular, generate very strong second harmonic, and hasdeveloped totally new crystals including the cesium-lithium-boratecrystal and crystals with substituted chemical compositions thereof ofthe present invention.

Prescribed crystals of the present invention are manufactured by beatingand melting a mixture of raw materials such as cesium carbonate(Cs₂CO₃), lithium carbonate (Li₂CO₃) and boron oxide (B₂O₃). Thereaction is expressed, for example, by the following formula:

Cs₂CO₃+Li₂CO₃+6B₂O₃—2CsLiB₆O₁₀2CO₂↑

or

Cs₂CO₃+Li₂CO₃+12H₃BO₃—2CsLiB₆O₁₀2CO₂↑+18H₂O↑

As a cesium-lithium-borate crystal substituted by an alkali metalelement or an alkali earth metal element (M) other than Cs and Li, thereis conceivable a crystal using an arbitrary alkali metal element otherthan Cs and Li, as expressed by the following formula:

Cs_(1−x)Li_(1−y)M_(x+y)B₆O₁₀

Examples of composition include a composition with 0<x≦0.01 when thealkali metal element (M) is Na (sodium), a composition with 0<x≦0.1 whenN is K (potassium), and a composition with 0<x≦1 when M is Rb (rubidium)as being within a suitable range from the point of view of manufactureand physical properties. It is needless to mention that a plurality ofalkali metal elements may be added.

By adding these alkali metal ions, it is possible to change therefractive index and phase matching angle, and to improve angularallowance and temperature allowance, and by simultaneously causing astructural change in crystal, a more stable crystal which is hard tocrack and free from becoming white-muddy is available.

In the case of the following formula:

Cs_(2(1−z))Li₂L_(z)B₁₂O₂₀

Ions of alkaline earth metals (L) such as Ba, Sr, Ca, Mg and Be areadded. It is needless to mention that a plurality of alkaline earthmetal elements may be added.

As in the case with alkali metals alone, addition of these alkalineearth metal ions permits changing the refractive index and the phasematching angle, and improvement of angular allowance and temperatureallowance. At the same time, it is possible to obtain a more stablecrystal by changing crystal structure.

In the present invention, the above-mentioned crystals can be used infrequency wave conversion or optical parametric oscillation (OPO). Inother words, the present invention permits achievement of an opticalapparatus provided with the above-mentioned crystals.

Now, the present invention will be described in further detail by meansof examples. It is needless to mention that the present invention is notlimited to the following examples.

EXAMPLE 1

Raw materials used were Cs₂CO₃, Li₂CO₃, and B₂O₃. These materials weremixed in a mol ratio of 1:1:6, and heated and melted to synthesizecrystals. The melting point of the crystal was 846° C.

There was obtained a transparent crystal measuring 30×25×25 mm afterapproximately two weeks' growth period using the top seed method andtemperature decrease, method to grow the crystal from the melt.

The chemical structural formula of the crystal obtained is CsLiB₆O₁₀according to the result of compsitional analysis such as ICP emissionspectral analysis and ICP mass spectroscopy. Measurement of the meltingpoint through a differential thermal analysis revealed a melting pointof 846° C. of this cesium-lithium-borate crystal. This crystal belongsto tetragonal system (space group I42d) according to the result of anX-ray structural analysis. Further, this cesium-lithium-borate crystalis transparent for the visible radiations, and it passes light ofwavelengths down to 180 nm.

The effective second-order nonlinear optical constant wasd_(eff)=4d_(KDP) according to the result of analysis by the powdermethod.

Further, a part of this cesium-lithium-borate crystal measuring 30×25×25mm was cut by the angle of phase matching, and polished. The polishedcrystal was then irradiated by 1.06 μm neodymium YAG laser light. Greenrays of 0.53 μm wavelength (the second harmonic), was obtainedefficiently.

A cesium-lithium-borate crystal measuring 10 diameter×20 mm was grown bythe revolving pull-up method (rate of revolution: 10 rpm; pull-up rate.:0.5 mm/h). This was confirmed to be the same as that described above.

EXAMPLE 2

A cesium-lithium-borate crystal (CsLiB₆O₁₀) comprising a stoichiometricchemical composition was manufactured by heating and melting a mixtureof cesium carbonate (Cs₂CO₃), lithium carbonate (Li₂CO₃) and boron oxide(B₂O₃) and the resultant cesium-lithium-borate crystal was grown by thetop seeded kyropoulos method in a five-zone furnace. FIG. 1 shows thestructure of the five-layer controlled growing furnace used for growingthe crystal. This five-layer controlled grossing furnace has a structurein which a cylindrical platinum crucible is installed in an uprightfive-stage resistance heating furnace capable of keeping a uniformtemperature in the furnace. The seed crystal of CsLiB₆O₁₀ was attachedto a platinum rod. The growing crystal was rotated at a rate of 15 rpmwith inverting the rotation direction in every three minutes.Temperature in the platinum crucible was kept at 848° C., the meltingpoint of the crystal. This permitted growth of a transparent andhigh-quality cesium-lithium-borate crystal, measuring 2.9 cm×2.0 cm×2.2cm, free from cracks in about four days. This represents a very shortperiod of growth as compared with the conventional growth of a nonlinearoptical borate crystal for frequency conversion. It is thus possible toeasily grow a cesium-lithium-borate crystal in a very short period oftime for growth by the application of the growing method of thecesium-lithium-borate crystal of the present invention.

According to the result of crystal structural analysis by means of aRigaku AFC5R X-ray diffraction apparatus, the cesium-lithium-boratecrystal was a tetragonal crystal belonging to the space group I42dsymmetrical group, with a crystal lattice constant of a=10,494 andc=8,939 Å, and a calculated density of 2.461 g/cm³. FIG. 2 shows thethree-dimensional structure of this cesium-lithium-borate crystal, whichsuggests a structure in which a cesium atom is present in a channel ofborate ring comprising boron and oxygen. It is clear that this crystalhas a structure quite different from that of LiB₃O₅ or CsB₃O₅ (both areor the rhombic), nonlinear optical crystal so far commonly used.

This cesium-lithium-borate crystal was transparent relative to a lighthaving a wavelength of from 180 nm to 2750 nm, according to the resultoI measurement of transmission spectra. FIG. 3 shows the transmissionspectrum in the short wavelength region. As is clear from FIG. 3, thecrystal had an absorption edge of 180 nm which was shorter by about 9 nmthan that of the conventional β-B_(a)B₂O₄ (189 nm).

The refractive index of this cesium-lithium-borate crystal was measuredby the prism method within a wavelength range of from 240 nm to 1,064nm. FIG. 4 shows a dispersion curve of the refractive index. In FIG. 4,“n_(o)” represents the refractive index for the normal light, and“n_(e)” indicates that for an abnormal light.

The approximation formula of refractive index (Sellmeier's equation) asavailable from this refractive index dispersion curve is as follows;$n_{o}^{2} = {2.19974 + \frac{1.18388 \times 10^{- 2}}{\lambda^{2} - {8.77048 \times 10^{- 3}}} - {8.52469 \times 10^{- 5}\lambda^{2}}}$$n_{e}^{2} = {2.05386 + \frac{9.4403 \times 10^{- 3}}{\lambda^{2} - {8.62428 \times 10^{3}}} - {7.82600 \times 10^{- 5}\lambda^{2}}}$

EXAMPLE 3

The relationship between the mixing ratio of B₂O₃ and temperature uponmanufacture of this cesium-lithium-borate crystal was determined. Themixing ratio of B₂O₃ was varied within a range of from 66.7% to 83.3%while keeping a constant mixing ratio of 1:1 between the initial rawmaterials Cs₂CO₃ and Li₂CO₃, and the melting point of the crystal wasdetermined by placing a sintered powder of the resultant mixture into adifferential thermal analyzer. FIG. 5 is a graph illustrating therelationship between the mixing ratio of B₂O₃ and temperature in thiscase. In FIG. 5, Cs₂CO₃ and Li₂CO₃ mixed at a ratio of 1:1 arerepresented by Cs₂O Li₂O. As is evident from FIG. 5, a CsLiB₆O₁₀ crystalwas stably available with a mixing ratio of B₂O₃ within a range of from66.7% to 81.8%. With a mixing ratio of B₂O₃ of under 66.7% CBOprecipitated along with CsLiB₆O₁₀ crystal, and with a mixing ratiowithin a range of from 81.8 to 83.8%, unknown crystals other thanCsLiB₆O₁₀crystal precipitated simultaneously, thus resulting in unstablemanufacture of the crystal. In the manufacture of thiscesium-lithium-borate crystal, therefore, the mixing ratio of B₂O₃, aninitial raw material, should preferably be kept within a range of from66.7% to 81.8%. The CsLiB₆O₁₀ crystal stably manufactured can meltcongruently at 848° C.

Since this cesium-lithium-borate crystal is a congruently meltingcrystal, it is possible to grow a high-quality crystal with a constantcomposition easily within a short period of time through adoption of theTop-seeded kyropoulos method, crystal pulling method or the flux method,as compared with the conventional β-BaB₂O₄ tending to easily cause phasetransition during growth from the melt and LiB₃O₅ requiring a longgrowth period for flux growth.

EXAMPLE 4

A cesium-lithium-borate crystal was manufactured by heating and melting12 kg mixture of cesium carbonate (Cs₂CO₃), lithium carbonate (Li₂CO₃)and boron oxide (B₂O₃) mixed at a ratio of 1:1:5.5 (B₂O₃ accounting for73.3%). The resultant cesium-lithium-borate crystal was largely grown bythe flux method in a five-zone furnace. As large crystal growth requiresa large temperature drop, the flux method is suitable. For the purposeof growing a large crystal, a large platinum crucible having a diameterof 20 cm and a height of 20 cm was employed. In this Example, the growthsaturation temperature was measured to be 845° C. The crystal was grownby reducing the growing temperature from 845° C. to 843.5° C. at a dailyrate of about 0.1° C. A large transparent crystal measuring 13 cm×12cm×10 cm and weighing about 1.6 kg could be grown in about 12 days. Inthis crystal growth, there was no unstable growth such as hopper growthobserved in the conventional growth of LiB₃O₅ crystal, proving a verystable growth.

EXAMPLE 5

By using the cesium-lithium-borate crystal (CsLiB₆O₁₀) of the presentinvention as a frequency converting nonlinear optical crystal of anNd:YAG laser, the second harmonic (SHG: wavelength: 532 nm) of theNd:YAG laser (wavelength: 1,064 nm) was generated.

FIG. 6 illustrates the relationship between the phase matching angle θof the crystal permitting second harmonic generation (SHC) and the inputlaser wavelength. In FIG. 6, the dotted line represents the calculatedvalues based on Sellmeier's equation for Type-I SHG, the solid line, thecalculated values for Type-II SHG by Sellmeier's equation, and the blackplots are observed values. The limit of SHG wavelength is 477 nm inType-I, and 640 nm in Type-II. As is clear from FIG. 6, for example, theType-I SHG of Nd:TAG laser beam having a wavelength of 1,064 nm shows anincident angle of 29.6° in calculation and about 30° in observation. TheType-I SHG of Nd:YAG laser beam having a wavelength of 532 nm gives anincident angle of 62.5° in calculation and 62° in observation. There issuggested a satisfactory agreement.

FIG. 7 is a graph illustrating the relationship between the walk-offangle and the wavelength for Type-I SHG obtained through calculationfrom Sellmeier's equation. In FIG. 7, the solid line representsCsLiB₆O₁₀ of the present invention, and the dotted line, theconventional β-BaB₂O₄.

Table 1 shows, for CsLiB₆O₁₀ of the present invention and theconventional β-BaB₂O₄ Type-I SHG, with an incident wavelengths of 1,064μm and 532 nm, calculated values of phase matching angle θ, effectivenonlinear optical coefficient d_(eff), angular allowance Δθ.L, spectralallowance Δλ.L, temperature allowance ΔT.L, walk-off angle, and laserdamage threshold value. As to the refractive index for β-BaB₂O₄necessary for calculating these values, those in literature released in“J. Appl. Phys. Vol. 62, D. Eimerl, L, Davis, S. Velsto, B. K. Grahamand A. Zalkin (1987), p. 1968.”

The effective nonlinear optical constant deff was derived fromcomparison with SHG of KH₂PO₄ (KDP) crystal. CsLiB₆O₁₀ has a crystalstructure identical with that of KDP.

The second-order nonlinear optical coefficient is expressed as d₃₆,where d₃₆(CLBO)=2.2×d₃₆(KDP)=0.95 pm/r, and its relationship withd_(eff) is d_(eff)=−d₃₆ sin θ sin 2φ. The value of d_(eff) wascalculated by means of this formula. A value of 0.435 pm/V was used asthe standard value of d₃₆ of KDP.

The angular allowance Δθ.L and the spectral allowance Δλ.L werecalculated in accordance with Sellmeier's equation.

The temperature allowance ΔT.L, which could not be obtained fromcalculation, was actually measured within a range of from 20° C. to 150°C.

TABLE 1 Fundamental Phase- Walk-off Damage wavelength matching d_(eff)Δθl Δλl ΔTl angle threshold (nm) Crystal angle (θ) (pm/V) (mrad-cm)(nm-cm) (° C.-cm) (deg) (GW/cm²) 1064 CLBO 29.6 0.47 1.02 7.03 1.78 26BBO 21 2.06 0.51 2.11 3.20 13.5 532 CLBO 62.5 1.01 0.49 0.13 9.4 1.83BBO 48 1.85 0.17 0.07 4.5 4.80 488 CLBO 77.9 1.16 0.84 0.09 0.98 BBO1.88 0.16 0.05 4.66

As is clear from Table 1, the CsLiB₆O₁₀ of the present invention has asmaller effective nonlinear optical constant as compared with theconventional BBO. The CsLiB₆O₁₀ of the invention is however larger inangular allowance, wavelength allowance and temperature allowance andsmaller in walk-off angle. The cesium-lithium-borate crystal of thepresent invention therefore permits more effective frequency conversionthan that with the conventional nonlinear optical crystal.

EXAMPLE 6

The fourth harmonic (4HG; wavelength: 266 nm) of Nd:YAG laser(wavelength: 1,064 nm) was generated by using the cesium-lithium-boratecrystal (CsLiB₆O₁₀) of the present invention in Nd:YAG laser. The secondharmonic (SHG) of Q-switch laser having a pulse width of 8 nanosecondswas employed as the incident light with a beam diameter of 4 mm andrepetition rate at 10 Hz. FIG. 8 is a graph illustrating therelationship between the energy output of the incident light SHG and theenergy output of 4HG, i.e., generation characteristics of the fourthharmonic. In FIG. 8, the solid line represents the CsLiB₆O₁₀ of thepresent invention, and the dotted line indicates β-BaB₂O₄. The samplelength was 9 mm for CsLiB₆O₁₀ and 7 mm for β-BaB₂O₄. As is evident fromFIG. 8, according as the energy of the incident light SHG becomeslarger, β-BaB₂O₄'s 4HG energy shows a tendency toward saturation,whereas CsLiB₆O₁₀ of the invention is proportional to the square of theincident energy, and in the high incident energy region with a highenergy of incident light, a 4HG output energy larger than that ofβ-BaB₂O₄ is available. The cesium-lithium-borate crystal of the presentinvention can therefore be used as a very excellent wavelengthconverting nonlinear optical crystal capable of generating ultravioletrays of a high output energy.

EXAMPLE 7

The fifth harmonic (5HG; wavelength: 213 nm) of Nd:YAG laser(wavelength: 1,064 nm) was generated by using the cesium-lithium-boratecrystal (CsLiB₆O₁₀) of the present invention in the Nd:YAG laser.

FIG. 9 is a graph illustrating the results of calculation of a frequencycapable of generating a sum frequency (ω₁+ω₂=ω₃) of two frequencies (ω₁and ω₂) in CsLiB₆O₁₀ of the invention, as derived from Sellmeir'sequation. The abscissa represents the light wavelength λ₁ correspondingto the frequency ω₁, and the ordinate represents the light wavelength λ₂corresponding to the frequency ω₂ and the light wavelength ω₃corresponding to the frequency ω₃. In FIG. 9, the region shadowed withoblique lines above the solid line is the region in which a sumfrequency can be generated. The dotted line represents the wavelength λ₃available as a result of a sum frequency. For example, assuming that anNd:YAG laser has a basic wave (wavelength: 1,064 nm) ω, then ω+4ω=5ω ispossible. In other words, the fifth harmonic can be generated by the sumfrequency of the basic wave and the fourth harmonic. Generation or thefifth harmonic by adding the second and third harmonics is howeverimpossible.

In FIG. 9, the black plots represent the wavelength available from sumfrequency of ω+4ω and that available from sum frequency of 2ω+3ω.Presence of only black plots representing ω+4ω in the slashed regionreveals that the fifth harmonic can be generated only from the sumfrequency of ω+4ω. As is evident from the dotted line (wavelength: λ₃)in FIG. 9, a wavelength of even under 200 nm can be generated from a sumfrequency by properly selecting wavelengths λ₁ and λ₂.

FIG. 10 illustrates a photograph of the beam pattern of the fifthharmonic of Nd:YAG laser generated with CsLiB₆O₁₀ of the presentinvention. FIG. 11 is a schematic view of crystal arrangement upongeneration of the second harmonic, the fourth harmonic and the fifthharmonic in this case Table 2 shows energy values of the individualfrequencies of CsLiB₆O₁₀ (LLBO) of the invention and the conventionalβ-BaB₂O₄(BBO). As is clear from Table 2, while 5ω available from theconventional β-BaB₂O₄(BBO) is 20 mJ, what is available from CsLiB₆O₁₀ ofthe invention is a higher output of 35 mJ. The cesium-lithium-boratecrystal of the present invention can therefore generate the fifthharmonic of a higher output than the conventional β-BaB₂O₄, and can beused as a nonlinear optical crystal for generating a very excellentfifth harmonic. The beam pattern obtained from FIG. 10 is closest tocircle, suggesting that it is possible to generate second harmonicsthroughout the entire beam. This is attributable to a larger angularallowance Δθ.L and temperature allowance ΔT.L of CLBO than BBO.

TABLE 2 Harmonic 2ω 4ω 5ω Crystal Power  (mJ) 350 110 35 CLBO Power (mJ)500  80 20 BBO

EXAMPLE 8

The output of wavelength of 488 nm of Ar laser was converted into asecond harmonic by means of the cesium-lithium-borate crystal of thepresent invention. Table 1 presented above shows, for the CsLiB₆O₁₀ ofthe invention and the conventional β-BaB₂O₄ relative to Type-I SHG withan incident wavelength of 488 nm, calculated values of phase matchingangle θ, effective nonlinear optical coefficient d_(eff), angularallowance Δθ.L, spectral allowance Δλ.L, and walk-off angle. The samecalculating methods as in the Example 5 were used for thesecalculations. As is clear from Table 1, CsLiB₆O₁₀ has a walk-off angleof 0.98° which is very close to the noncritical phasematching. Thisdemonstrates that the cesium-lithium-borate crystal of the inventiongives a very high conversion efficiency as compared with theconventional β-BaB₂O₄.

EXAMPLE 9

The cesium-lithium-borate crystal of the present invention was used foroptical parametric oscillation (OPO).

Optical parametric oscillation (OPO) is a process of wavelengthconversion comprising exciting a nonlinear polarization within anonlinear optical crystal with a laser beam, thereby dividing the energyof the excited beam through nonlinear oscillation of polarized electronsinto a signal light and an idler light.

Because of the possibility to tune a wavelength region within a widerange, a wider application of OPO is expected. The cesium-lithium-boratecrystal of the invention, having a relatively large effective nonlinearoptical coefficient, can supply a longer crystal length, because of thelasiness of growing a large crystal, and further to a larger powerdensity of the excited beam because of the high laser damage thresholdvalue, thus providing excellent characteristics as an OPO crystal

FIG. 12 is a phase matching tuning curve diagram illustrating therelationship between the wavelengths of the signal light produced inType-I With excited light wavelengths of 213 nm, 266 nm, 355 nm and 532nm, and the corresponding phase matching angles. FIG. 13 is a phasematching tuning curve diagram illustrating the relationship between thewavelength of the signal light produced in Type-II with a wavelength ofthe excited light of 532 nm and the corresponding phase matching angle.As is clear from FIGS. 12 and 13, the cesium-lithium-borate crystal ofthe present invention exhibits excellent properties also as an OPOcrystal.

Particularly OPO based on excitation with the fourth harmonic(wavelength: 266 nm) of Nd:YAG laser gives a variable-wavelength laserbeam near 300 nm. This has been impossible in the conventional β-BaB₂O₄because of the small angular allowance Δθ.L and the large walk-off.

EXAMPLE 10

An Rb (rubidium)-substituted cesium-lithium-borate crystalCs_(1−x)LiRb_(x)B₆O₁₀ was manufactured in the same manner as in theExample 2.

According to the results of evaluation of the resultant crystal by thepowder X-ray diffraction method, as shown in FIG. 14, particularly, theinterval between reflection peak of (312) plane and reflection peak of(213) plane becomes gradually arrower by sequentially increasing theamount of added Rb from x=0.2, to 0.5 and then 0.7 to the X-raydiffraction pattern of the sample (Rb, X=0) not added with Rb. Thisshows that Cs and Rb enter into the crystal at an arbitrary ratio. Thecrystal added with Rb arbitrarily is the same tetragonal crystal as CLBOnot added with Rb, and the lattice constant varies accordingly.

Since it is possible to add Rb ions in an arbitrary amount, it ispossible to change the refractive index of the crystal, and this revealsthe possibility to improve the phase matching angle, angular allowanceand temperature allowance.

Similarly, crystals with amounts (x) of added Rb of under 0.1 weremanufactured. It was confirmed as a result that the stability of crystalstructure vias morg satisfactory.

EXAMPLE 11

Crystals were manufactured in the same manner as in the Example 10except that K or Na was added in place of Rb. Availability ofhigh-quality crystals was confirmed with a constituent ratio (x) ofunder 0.1 for κ (potassium), and under 0.01 for Na (sodium).

Crystals in which K or Na was coexistent with Rb were also manufactured.In this case, in a composition:

Cs₁−xLi₁yRb_(x)(Na,K)_(y)B₆O₁₀,

a more stable crystal was obtained with 0<x≦1 and 0<y<0.1.

EXAMPLE 12

Crystals were manufactured in the same manner as in the Example 10except that an alkaline earth metal element was added in place of analkali metal.

For example, in the case of a composition Cs_(2(1−z))Li₂Ba_(z)B₁₂O₂₀ itwas confirmed that a stable crystal is available with 0<z≦0.1.

According to the present invention, as described above in detail a novelcesium-lithium-borate crystal and substituted cesium-lithlium-boratecrystals are provided. These crystals permit conversion of frequency,has a high converting efficiency, and wide temperature allowance andangular allowance, and can be used as a high-performance frequencyconverting crystal. Furthermore, the CsLiB₆O₁₀ crystal has a low meltingpoint of 848° C., and because of the congruency of CsLiB₆O₁₀ crystal, itis possible to easily grow a large high-quality crystal having a stablecomposition by the application of the melt methods based on theTop-seeded kilgpoulos method, crystal pulling method or the flux method.

What is claimed is:
 1. A cesium-lithium-borate crystal having a chemicalcomposition consisting of CsLiB₆O₁₀.
 2. A method for manufacturing acrystal as claimed in claim 1, which comprises forming a mixturecomprising a source of Cs, Li, B and O, and heating and melting themixture to a temperature sufficient to form said crystal.
 3. The methodfor manufacturing a crystal according to claim 2, which comprises a stepof growing the crystal by the melt method.
 4. The method formanufacturing a crystal according to claim 2, which comprises a step ofgrowing the crystal by the flux method.
 5. A cesium-lithium-boratecrystal having a chemical composition consisting ofCs_(1−x)Li_(1−y)M_(x+y)B₆O₁₀ wherein M is at least one alkali metalelement other than Cs and Li, and wherein x and y satisfy therelationship of 0<x<1 and 0<y<1.
 6. The crystal according to claim 5,wherein M is at least one alkali metal element selected from the groupconsisting of Na, K and Rb.
 7. A method for manufacturing a crystal asclaimed in claim 5, which comprises forming a mixture comprising asource of Cs, Li, B, O, and M, wherein M has the same meaning as definedin claim 5, and heating and melting the mixture to a temperaturesufficient to form said crystal.
 8. The method for manufacturing acrystal according to claim 7, which comprises a step of growing thecrystal by the melt method.
 9. The method for manufacturing a crystalaccording to claim 7, which comprises a step of growing the crystal bythe flux method.
 10. A frequency converting apparatus which contains acrystal according to claim
 5. 11. The apparatus according to claim 10,which generates a second, third, fourth or fifth harmonic of a laser.12. An optical parametric oscillator which contains a crystal accordingto claim
 5. 13. A cesium-lithium-borate crystal having a chemicalcomposition consisting of Cs_(2(1−z))Li₂L_(z)B₁₂O₂₀, wherein L is atleast one alkaline earth metal ion, Mg or Be and wherein 0<z<1.
 14. Thecrystal according to claim 13, wherein L is at least one alkaline earthmetal ion selected from the group consisting of Ba, Sr and Ca.
 15. Amethod for manufacturing a crystal as claimed in claim 13, whichcomprises forming a mixture comprising a source of Cs, Li, B, O, and L,wherein L has the same meaning as defined in claim 13, and heating andmelting the mixture to a temperature sufficient to form said crystal.16. The method for manufacturing a crystal according to claim 15, whichcomprises a step of growing the crystal by the melt method.
 17. Themethod for manufacturing a crystal according to claim 15, whichcomprises a step of growing the crystal by the flux method.
 18. Afrequency converting apparatus which contains a crystal according toclaim
 13. 19. The apparatus according to claim 18, which generates asecond, third, fourth or fifth harmonic of a laser.
 20. An opticalparametric oscillator which contains a crystal according to claim 13.